There have long been methods of two-dimensional latitude/longitude position location systems using radio signals. In wide usage have been terrestrial systems such as Loran C and Omega, and a satellite-based system known as Transit. Another satellite-based system enjoying increased popularity is the Global Positioning System (GPS).
Initially devised in 1974, GPS is widely used for position location, navigation, survey, and time transfer. The GPS system is based on a constellation of 24 on-orbit satellites in sub-synchronous 12 hour orbits. Each satellite carries a precision clock and transmits a pseudo-noise signal, which can be precisely tracked to determine pseudo-range. By tracking 4 or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996.
GPS has revolutionized the technology of navigation and position location. However in some situations, GPS is less effective. But because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of −160 dBw as received by an omni-directional antenna). Thus the GPS signal is marginally useful or not useful at all in the presence of blockage or inside a building.
As an alternative or supplement to GPS, broadcast television signals can be used to determine the position of a user device. Techniques for determining the position of a user device using the American Television Standards Committee (ATSC) digital television (DTV) signal are disclosed in U.S. Pat. No. 6,861,984, “Position Location using Broadcast Digital Television Signals,” the disclosure thereof incorporated by reference herein in its entirety. Techniques for determining the position of a user device using the European Telecommunications Standards Institute (ETSI) Digital Video Broadcasting (DVB) signal are disclosed in U.S. Non-provisional U.S. Pat. No. 7,126,536, “Wireless Position Location Using the Japanese ISDB-T Digital TV Signals,” the disclosure thereof incorporated by reference herein in its entirety. Techniques for determining the position of a user device using the Japanese Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) signal are disclosed in U.S. Pat. No. 6,952,182, “Position Location using Terrestrial Digital Video Broadcast Television Signals,” the disclosure thereof incorporated by reference herein in its entirety. Techniques for determining the position of a user device using the NTSC (National Television System Committee) analog television (TV) signal are disclosed in U.S. Pat. No. 6,559,800 and U.S. Pat. No. 6,522,297, the disclosures thereof incorporated by reference herein in their entirety.
Each of these television signals includes components that can be used to obtain a pseudo-range to the transmitter of the television signal. When multiple such pseudo-ranges are known, and the locations of the transmitters are known, the position of the user device can be determined with accuracy. Suitable components within the ATSC digital television signal include synchronization codes such as the Field Synchronization Segment within an ATSC data frame and the Synchronization Segment within a Data Segment within an ATSC data frame. Suitable components within the ETSI DVB and ISDB-T digital television signals include scattered pilot carriers. Suitable components within the NTSC analog television signal include the horizontal synchronization pulse, the horizontal blanking pulse, the horizontal blanking pulse and horizontal synchronization pulse taken together, the ghost canceling reference signal, the vertical interval test signal, and other chirp-type signals.
One approach to providing additional navigation information concerns the use of Doppler measurements of wireless signals. When a user is moving, due to the Doppler effect, the observed frequency of a wireless signal shifts in proportion to the user velocity, which can be converted to a position relative to a known starting point. This concept of Doppler positioning is well known and has been tried in Transit, one of the earliest satellite navigation systems. However, because satellites are moving very fast (4 km/s for GPS satellites), and are more sensitive to user vertical movement, it is rather hard to detect user horizontal motion—the primary concern of pedestrian positioning—from the Doppler shift measured from satellite signals.